Fuel assembly with short fuel units

ABSTRACT

A fuel assembly for a boiling water reactor comprising a plurality of fuel units, stacked on top of each other, each of which comprising a plurality of fuel rods extending vertically between a top tie plate and a bottom tie plate, and means for keeping the fuel elements together. The fuel elements are surrounded by a fuel channel with a substantially square cross section. At least two of the fuel units differ from each other in regard to fuel distribution or free flow area.

TECHNICAL FIELD

The present invention relates to a fuel assembly for a boiling waterreactor comprising a plurality of fuel rods extending between a top tieplate and a bottom tie plate and being surrounded by a fuel channel witha substantially square cross section.

BACKGROUND OF THE INVENTION

In a boiling water nuclear reactor, moderated by light water, the fuelis in the form of fuel rods, each of which contains a stack of pelletsof a nuclear fuel arranged in a cladding tube. A fuel bundle comprises aplurality of fuel rods arranged in parallel with each other in a certaindefinite, normally symmetrical pattern, a so-called lattice, and isretained at the top by a top tie plate and at the bottom by a bottom tieplate. To keep the fuel rods spaced from each other and prevent themfrom bending or vibrating when the reactor is in operation, a number ofspacers are distributed along the fuel bundle in the longitudinaldirection. A fuel assembly comprises one or more fuel bundles, each ofwhich extends along the main part of the length of the fuel assembly,surrounded by a substantially square fuel channel.

A core in a boiling water reactor comprises several hundred fuelassemblies arranged vertically in the core in a certain spacedrelationship to each other. The fuel assemblies are arranged in asymmetrical lattice with each fuel assembly included in two rows of fuelassemblies perpendicular to each other. The core also comprises controlrods with four blades, extending perpendicularly to each other from acentral part and forming a right-angled cross. The control rods arearranged with each of their blades between two fuel assemblies locatedin the same row, such that each control rod together with four fuelassemblies arranged around its fuel blades forms one unit.

The core is immersed into water which serves both as coolant and asneutron moderator. During operation, the water flows from below andupwards through the fuel assembly, whereby part of the water istransformed into steam. The percentage of steam increases towards thetop of the fuel assembly. In the lower part of the fuel assembly, thecoolant is in singlephase state and in the upper part thereof intwo-phase state. This difference between the upper and lower parts givesrise to special problems which must be taken into account when the fuelis formed, such as the following:

the pressure drop is several times higher in the upper part of the fuelassembly than in the lower part thereof, which, among other thingsincreases the risk of thermohydraulic instability;

there is a risk of dryout in the upper part of the fuel assembly but notin the lower part thereof; and

the neutron moderation is less effective in the upper part, which leadsto the fuel not being burnt up as quickly in the upper part of the fuelassembly as in the lower part thereof.

Therefore, it is desirable to achieve a fuel assembly which in a simplemanner can be designed where the upper part of the fuel assembly differsfrom the lower part, and where the fuel distribution and the free flowarea may be varied in the axial direction to obtain optimum conditions.

One problem with conventional fuel assemblies for boiling water reactorsis that they are not sufficiently flexible and that it is thereforedifficult to give them different designs in the upper and the lowerpart. It is also difficult to obtain an optimum fuel distribution in theaxial direction.

A fuel bundle for a boiling water reactor is normally about four metersin length and has a width which varies between 0.05 and 0.2 mm. Theconsiderable length of the fuel bundle in relation to the width thereofentails difficulties in the manufacture, transport, handling, andstorage of the fuel bundles. Another disadvantage with full-length fuelrods is that they contain large quantities of uranium and harmfulfission products. This means that fuel damage to a fuel rod may haveserious consequences since considerable quantities of uranium andfission products risk leaking out into the coolant.

Experience shows that, for example in connection with repairs andservice of a nuclear reactor, debris may enter, for example metal chips,which then move with the water which circulates through the core. It hasbeen found that these debris may give rise to abrasion damage to thecladding tube. The abrasion damage normally arises on a level with thespacers because the debris adhere to the spacers and remain there andsubject the cladding tube to wear. Penetrating water then tends to giverise to considerable secondary damage, often far away from the primarysite of the damage.

British patent specification No. 1 403 491 shows a fuel element for anuclear reactor containing a plurality of short fuel units, each oneconsisting of a plurality of fuel rods arranged in parallel with eachother between a top tie plate and a bottom tie plate. The fuel rods arealso supported by intermediate spacers. The fuel units are fitted onto asupporting tube in such a way that the bottom plate of one fuel unitrests on the top plate of the other, and that the fuel rods in each fuelunit are parallel to the fuel rods in the other fuel units. Thesupporting tube with the fuel units extends through the whole fuelassembly. The fuel units have a substantially circular cross sectionalong their longitudinal axis. This fuel element is intended to be usedin a heavy-water moderated nuclear reactor. A heavy-water moderatednuclear reactor comprises a plurality of pressure-supporting coolingchannels. During their operating period, the fuel elements are insertedinto these cooling channels.

Another embodiment of short fuel units is known from Canadian patentspecification No. 1 250 966. Each fuel unit is sufficiently short toeliminate the need for intermediate spacers. Also these fuel units areprimarily intended to be used in a heavy-water moderated reactor,especially with pressure tubes, and have a substantially circular crosssection along the whole of their length.

The need of a flexible fuel assembly for a boiling water reactor isconnected with the problems which arise due to the water—which is boththe moderator and the coolant—boiling and the special requirementstherefore made on the upper and lower parts of the fuel assembly. Theseproblems have been known for a long time (since the early 60's).

Using short fuel units in a heavy-water moderated reactor has been knownsince the childhood of nuclear engineering. The conditions for aheavy-water moderated and a light-water moderated nuclear reactor are,however, very different; for example, a heavy water moderated reactorhas a low burnup and a high linear power density compared with alight-water moderated reactor which has a high burnup but a lower linearpower density. A decisive difference in comparison with a light-watermoderated reactor of boiling-water type is that the moderation iscompletely dominated by a large volume of separate moderator water,which is not allowed to boil in current heavy-water reactors. This meansthat the moderation is essentially constant in the axial direction alongthe fuel units.

Nor is the cooling water allowed to boil in current heavy-waterreactors. For that reason, there is not the same need of an axialvariable fuel design.

SUMMARY OF THE INVENTION

The invention provides a fuel assembly for a boiling water reactor,moderated by light water, wherein:

it is simple to give the fuel assembly different designs in the upperand the lower part;

flexibility is achieved for optimization of fuel distribution in theaxial direction, both initially and in partially burnt-up state;

flexibility is achieved for lattice optimization both in the axialdirection and in the radial direction;

the consequences in case of cladding damage, in the form of leakage offuel and fission products, are smaller than for a traditional fuel;

the risk of abrasion damage caused by debris in the cooling water issmaller than for a traditional fuel; and

the manufacture, transport, handling, and storage of the fuel assemblyare simplified.

According to the invention, this is achieved by dividing the fuelbundles into a plurality of short fuel units. Compared with heavy-waterreactors and the applications known there-from, light-water boilingreactors place essentially different requirements on the design of thefuel:

the fuel units shall constitute a constructive part of a retained fuelassembly with associated fuel channels with a substantially square crosssection;

a burnup several times higher must be attained without the fission gaspressure in the short fuel rods becoming too high;

the thermohydraulic requirements during boiling must be satisfied byactions to promote the utilization of the coolant in the upper part ofthe fuel assembly, and by limiting the pressure drop there; and

the nuclear requirements during boiling in combination with light-watermoderation must be satisfied by ensuring moderation internally in theassembly, and by adapting the distribution of fissile material axiallyin the assembly.

What characterizes a fuel assembly according to the invention willbecome clear from the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a fuel assembly according to the invention.

FIG. 2 shows a section II-II′ through the fuel assembly in FIG. 1.

FIGS. 2a and 2 b show other embodiments of a fuel assembly according tothe invention.

FIGS. 3a-3 c show examples of what is intended by a substantially squarecross section.

FIG. 4a shows a section IVA-IVA′ through the fuel assembly in FIG. 1.

FIG. 4b shows a spring arrangement for retaining the fuel units.

FIGS. 5a and 5 b show a supporting rod for retaining the fuel units.

FIGS. 6a and 6 b show a clutch spring for retaining the fuel units.

FIG. 7 shows a bottom tie plate in a section VII-VII′ through the fuelassembly in FIG. 1.

FIG. 8 shows in a section VIII-VIII′ in FIG. 7 how the fuel rods in thefuel units are attached to the top tie plate and the bottom tie plate.

FIG. 9a shows an example of how a retaining fuel rod may be fixed to thetop tie plate and/or the bottom tie plate.

FIG. 9b shows a section IXB-IXB′ through the attachment device in FIG.1.

FIG. 10 shows an additional example of how a retaining fuel rod may befixed to the top tie plate and/or the bottom tie plate.

FIGS. 11a, 11 b and 11 c show different embodiments of where the risk ofabrasion damage in the region nearest below the top tie plate has beenreduced.

FIG. 12 schematically shows an embodiment of a fuel assembly accordingto the invention, where the fuel rods in the lower fuel units have alarger diameter than the fuel rods in the upper fuel units.

FIG. 13 schematically shows an additional embodiment of a fuel assemblyaccording to the invention, where the number of fuel rods in the lowerfuel units is larger than the number of fuel rods in the upper fuelunits.

FIGS. 14a-14 d schematically show how the height of the fuel units mayvary in a fuel assembly and between the fuel assemblies.

FIGS. 15a-15 c schematically show an additional embodiment of a fuelassembly according to the invention where the fuel units in the upperand lower parts of the fuel assembly have different lattices.

FIG. 16 shows a cross section through a fuel assembly with 12×12 fuelrod positions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a fuel assembly according to the invention. The fuelassembly comprises an upper handle 1, a lower end portion 2, and aplurality of fuel units 3 stacked one above the other. Each fuel unitcomprises a plurality of fuel rods 4 arranged in parallel and in adefinite space relationship to each other in a given lattice, and a toptie plate 5 and a bottom tie plate 6 for attachment of the fuel rods intheir respective positions in the lattice. The fuel units 3 are stackedon top of each other in the longitudinal direction of the fuel assemblyand they are stacked in such a way that the top tie plate 5 in one fuelunit is facing the bottom tie plate 6 in the next fuel unit in thestack, and such that the fuel rods in all the fuel elements are parallelto each other. A fuel rod 4 comprises fuel in the form of a stack ofpellets 7 of uranium arranged in a cladding tube 19.

FIG. 2 shows a section II-II′ through the fuel assembly in FIG. 1. Thefuel assembly is enclosed in a fuel channel 8 with a substantiallysquare cross section. The fuel channel is provided with a hollow supportmember 9 of cruciform cross section which is secured to the four wallsof the fuel channel. In the central channel 14 formed by the supportmember 9, moderator water flows. The fuel channel with the supportmember surrounds four vertical channel-formed parts 10, so-calledsub-channels, with an at least substantially square cross section. Thefour sub-channels each contain a stack of fuel units. Each fuel unitcomprises 24 fuel rods 4 arranged in a symmetrical 5×5 lattice.

The fuel assembly in FIG. 2 comprises 10×10 fuel rod positions. By afuel rod position is meant a position in the lattice. All the fuel rodpositions in the lattice need not be occupied by fuel rods. In certainfuel assemblies, a number of fuel rods have been replaced by one or morewater channels. The introduction of a water channel changes the numberof fuel rods but not the number of fuel rod positions.

FIG. 2a shows another embodiment of a fuel assembly according to theinvention. The figure shows a horizontal section through the fuelassembly which is provided with an internally arranged vertical channel14 a through which water is conducted in a vertical direction from belowand upwards through the assembly. The channel 14 a is surrounded by atube 9 a with a substantially square cross section. The fuel units arekept in position by being fitted onto the tube which surrounds thevertical channel.

FIG. 2b shows an additional embodiment of a fuel assembly according tothe invention. The figure shows a horizontal section through the fuelassembly which is provided with two centrally arranged vertical waterrods 9 a and 9 c through which water is conducted from below and upwardsthrough the assembly. The water rods have a diameter which is somewhatlarger than the diameter of the fuel rods and are designed with asubstantially circular cross section. The fuel units are kept inposition by being fitted onto the water rods.

FIGS. 3a-3 c show examples of what is meant by a substantially squarecross section. The fuel assembly in FIG. 3a has a reduced corner portion11. The fuel assembly in FIG. 3b has two reduced corner portions. Thefuel assembly in FIG. 3c has four reduced corner portions. The reductionof a corner portion reduces the number of fuel rods in the fuel unit byone fuel rod compared with a fuel unit without a reduced corner portion.

In a boiling water reactor, cooling water flows upwards through thefuel, whereby part of the water is transformed into steam. This resultsin a greater pressure drop in the upper part of the fuel assembly thanin the lower part thereof. This difference gives rise to a force whichtends to raise the fuel upwards. In conventional fuel assemblies, thefuel bundles are kept in position because of their weight. In a fuelassembly with short fuel units there is a risk of the upper fuel unitsbeing raised upwards by these forces.

To prevent certain fuel units from being pressed upwards, a spring means12 is arranged in the upper part of the fuel assembly. FIG. 4a shows asection IVA-IVA′ through the fuel assembly in FIG. 1. FIG. 4b shows inmore detail the appearance of a spring means 12 in FIG. 1. The springmeans 12 comprises a spiral spring 13 arranged in a slit in the supportmeans 9 around the central channel 14. The spring 13 is provided withfour radially extending arms 15, each of which presses down a stack offuel units. This arrangement gives each stack of fuel units a freedom togrow in relation to the fuel channel independently of how the otherstacks grow. Two types of growth occur in the fuel units, namely,thermal growth and irradiation growth.

FIG. 5a shows another embodiment where the fuel units 3 are kept inposition by being fitted onto a common supporting member. The commonsupporting member may, for example, be a tube 50 a which conductsnon-boiling water. The advantage of using one or more water-filled tubesas a common supporting element is that non-boiling water may be movedinto the central parts of the fuel assembly and hence attain an improvedmoderation. The common supporting member may, for example, comprise aplurality of joined-together short fuel rods (50 b) as shown in FIG. 5b.The advantage of allowing the supporting member to comprise a pluralityof short fuel rods instead of one long fuel rod is that the previouslymentioned risks in case of fuel damage are reduced.

FIGS. 6a and 6 b show another arrangement for keeping the fuel units inplace. Two fuel units are connected to each other by four connectingsprings 51 arranged between the top tie plate of the lower fuel unit andthe bottom tie plate of the upper fuel unit. The connecting springcomprises an attachment loop 52, which may, for example, be of Inconelor some other nickel-base alloy. The connecting springs are easy to opensuch that the fuel units may change be rearranged or replaced duringrefuelling. The springs cannot be unintentionally opened when the bundlestands in the fuel channel or is being raised.

FIG. 7 is a section VII-VII′ through the fuel assembly in FIG. 1 andshows an example of a bottom tie plate 6. FIG. 8 shows the bottom tieplate in a section D—D in FIG. 7. The bottom tie plate comprises anorthogonal latticework composed of tubular sleeves 16, 17 and surroundedby a frame 18. The function of the frame 18 is to guide the fuel unitswhen charging the fuel, keep the fuel rods at a certain distance fromthe fuel channel 8, and to scrape off water from the walls of the fuelchannel, especially in the upper part of the channel. The frame isprovided with guiding vanes l9 a, the function of which is two-fold,namely, to facilitate the introduction of the fuel unit into thecladding tube, and to increase the mixing of cooling flow. The sleevesare of two different types, namely, fixing sleeves 16 in which the fuelrods 4 are fixed, and supporting sleeves 17 which support and fix thefixing sleeves. The fixing sleeves have the same or almost the samediameter as the cladding tube of the fuel rods. The fixing sleeves arearranged in a symmetrical 5×5 lattice which corresponds to the latticeof the fuel rods, and the supporting sleeves are arranged between thefixing sleeves to support these.

The supporting sleeves 17 may be provided with mixing vanes 19 b toincrease the mixing of the coolant flow. The mixing should primarily beperformed in the upper part of the fuel assembly, where the risk ofdryout is greatest. The fuel assembly preferably comprises two types offuel units, of which one type has bottom tie plates with mixing vanesand the other type has bottom tie plates with no mixing vanes. The fuelunits whose bottom tie plates have mixing vanes are arranged in theupper part of the fuel assembly and those without mixing vanes arearranged in the lower part of the fuel assembly.

The top tie plate 5 may be designed as the bottom tie plate describedabove. The frame of the top tie plate is suitably provided with amarking for identification of the respective fuel unit. The top tieplate shall also be capable of being gripped by a lifting tool.

FIG. 8 shows how the fuel rods 4 are attached to the top tie plate 5 andto the bottom tie plate 6. In the lower part of the fuel rod 4, a bottomplug 20 is arranged, the free end of which is inserted into the fixingsleeve 16 in the bottom tie plate 6. In the uppermost part of the fuelrod, a top plug 21 is arranged, the free end of which is inserted into afixing sleeve 22 in the top tie plate 5.

During the burnup of the nuclear fuel, fission gases contained in thefuel rod are released. To prevent the pressure on the cladding frombecoming too great, an expansion space for the fission gases is needed.The bottom plug 20 is provided with a cavity 23 to receive fissilegases, and that part of the bottom plug which faces the uranium pelletshas an opening between the cavity and the remainder of the fuel rod. Inthe upper part of the fuel rod, the stack of uranium pellets endssomewhat below the top plug 21 which is provided with a cylindricalrecess 25, the opening of which faces the uranium pellets. The spacebetween the top plug and the uranium pellets and the space in the topplug may be utilized for expansion of the fissile gases.

The uranium-free parts of the fuel rods give a reduced neutronabsorption, which leads to an increase of the effect in the uraniumpellets nearest the top and bottom plugs. To reduce the effect and tofurther increase the space for the fission gases, uranium pellets withholes may preferably be used nearest the top and bottom plugs. It mayalso be suitable to give these pellets a lower enrichment. It isimportant that the emission of fission gases be kept at a low level,such that the required fission space becomes as small as possible. Thisis achieved by a low linear rod load (kW/m) which is made possible by alarge number of fuel rods (96 in the embodiment) in the highly loadedcross section of the assembly. An additionally larger number of rods mayalso be advantageous. The number of fuel rod positions should at leastbe 80, preferably more than 90, for the fission gas emission to besufficiently low to be taken care of in the short fuel units. Thefission gas emission may be further reduced by additions to the fuelpellets.

A fuel unit comprises a small number of, for example two, retaining fuelrods which are fixed to the top tie plate and the bottom tie plate. Theretaining fuel rods retain the fuel unit such that the other fuel rodsare kept in position. FIG. 9a shows how a retaining fuel rod 4 a may befixed to the fixing sleeve 16 of the bottom tie plate with a cleavingrivet 26. FIG. 9b shows a section IXB-IXB′ in FIG. 9a. FIG. 10 shows howa retaining fuel rod 4 b may be fixed to the fixing sleeve 16 with ascrew joint 27.

In a conventional fuel assembly, with full-length fuel rods, which areretained by a plurality of spacers along their axial length, abrasiondamage normally arises on a level with the spacers because debrisadhering thereto remain and wear holes in the cladding. Because nospacers are needed in a fuel assembly according to the invention, therisk of abrasion damage to the fuel is reduced. However, a risk ofabrasion damage remains in the region below the top tie plate. FIGS.11a-11 c show different alternatives for reducing the risk of claddingdamage caused by abrasion in this region. Because the rods need not bedrawn through spacers during mounting, a larger outer diameter may beallowed in this region, for example by a wear-resistant coating.

FIG. 11a shows a top plug 33 which has an upper solid part 34, forconnection to the fixing sleeve 22 in the top tie plate, and a lowersolid 35. The lower part 35 is longer than the upper part 34. The lowerpart is arranged in the region with the greatest risk of abrasiondamage.

FIG. 11b shows a top plug 28 which has an upper solid part 29 and alower hollow part 30. The lower part 30 is longer than the upper part 29and is provided with a coating 31 which protects against abrasiondamage, for example zirconium oxide or aluminium oxide.

FIG. 11c shows a fuel rod, the cladding tube 19 of which in its upperpart, where the risk of abrasion damage is greatest, is provided with acoating 32 protecting against abrasion damage.

For several different reasons, it is desirable to reduce the amount ofuranium in the upper part of the fuel assembly in a fuel assemblyintended for a boiling water reactor. One reason is that the highpercentage of steam in the upper part of the fuel assembly leads todeteriorated neutron moderation, which results in the fuel not beingburnt up as quickly in the upper part of the fuel assembly as in thelower part thereof. Another reason is that a reduction of the quantityof uranium in the upper part of the fuel assembly gives an improvedshut-down margin. A consequence of the reduction of the quantity ofuranium is that the free flow area increases, which leads to a reductionof the pressure drop in the upper part of the fuel assembly, and,therefore, the risk of thermohydraulic instability in the fuel assemblydecreases.

In a fuel assembly according to the invention, the quantity of uraniumin different parts of the assembly may be varied in a simple manner. Afuel assembly may comprise fuel units with different numbers of fuelrods, different lattice configurations, and different fuel roddiameters. FIG. 12 shows a fuel assembly 39 which comprises fuel units(40, 41) of two different types, of which the first type contains fuelrods with a first diameter and the other type contains fuel rods with asecond diameter. The first type of fuel units 40 is arranged in thelower part of the fuel assembly, and the second type of fuel units 41 isarranged in the upper part of the fuel assembly. The fuel rods in thelower fuel units 40 have a diameter which is larger than that of thefuel rods in the upper fuel units 41.

FIG. 13 shows a fuel assembly 42 where the number of fuel rods in thefuel units 43 in the lower part of the fuel assembly is larger than thenumber of fuel rods in the fuel units 44 in the upper part of the fuelassembly. The number of axial zones with different rod diameters ordifferent number of rods may, of course, be greater than the two shownin the examples. It is also possible to have different rod diameterswithin the fuel units to attain optimum properties in the cross section.

A fuel assembly according to the invention may comprise fuel units withdifferent height. FIG. 14a shows a fuel assembly which comprises eightequally long fuel units 60. FIG. 14b shows a fuel assembly whichcomprises eight fuel units with two different heights. The uppermostfour fuel units 61 are shorter than the lowermost four fuel units 62.Since the top tie and bottom tie plates give rise to turbulence of thecooling water, it is advantageous, from the point of view of dryout, tohave more top tie and bottom tie plates in the upper part of the fuelassembly than in the lower part thereof, which is achieved in thisembodiment. FIG. 14c shows ten equally high fuel units 63. FIG. 14dshows five equally long fuel units 64, each one comprising a spacer 65which keeps the fuel rods spaced-apart from each other and prevents themfrom bending or vibrating when the reactor is in operation.

FIG. 15a shows an example of a fuel assembly where the fuel units 69 inthe upper part and the fuel units 68 in the lower part of the fuelassembly have different lattices. FIG. 15b shows a cross section throughthe fuel unit 69, and FIG. 15c shows a cross section through the fuelunit 68. The number of fuel rods is larger in the fuel units in thelower part of the fuel assembly than in the fuel units in the upper partthereof. It is also possible to omit rods in occasional latticepositions, preferably in the uppermost fuel units.

A fuel assembly according to the invention may also be optimized by theenrichment of uranium in the fuel rods varying between the differentfuel units. Fuel units in the upper part of the fuel assembly may, forexample, have a lower enrichment than fuel units in the lower part ofthe fuel assembly. The occurrence of burnable absorbers, for examplegadolinium, may also vary between the fuel units.

Recently, the development has gone towards fuel assemblies with narrowerfuel rods which are more in number. However, there is a limit to hownarrow the rods may be if they are to have a length of about fourmeters. If the rod is too narrow, mechanical difficulties arise whichmay become very difficult to solve. A solution to these problems is tomanufacture short fuel units. FIG. 16 shows a cross section of a fuelassembly with 12×12 fuel rod positions. The rod diameter is about 8 mm.With a plurality of rods, the linear load and hence the fission gasemission are reduced. The need of space for fission gas in the shortrods is thus reduced, which facilitates such a design.

A fuel assembly with short fuel units has several advantages comparedwith a traditional fuel assembly with full-length fuel rods. Oneconsiderable advantage is the flexibility provided in designing the fuelassembly. This means that the fuel assembly in a simple manner may beoptimized both in the axial direction and in the radial direction, forexample with respect to lattices and fuel distribution. In connectionwith refuelling, certain fuel units may be replaced and certain may beallowed to remain in the fuel assembly. The fuel units which are allowedto remain in the fuel assembly may be given a new position. In this way,the service life of the fuel assembly increases.

If the height of the fuel units is sufficiently low, no spacers areneeded, which is an advantage since the spacers increase the risk ofabrasion damage. It is also easy to design the upper end of the rodswith special abrasion protection in the sensitive region below the toptie plates. The consequence of abrasion damage or other cladding damageis reduced as the length of the fuel rods is reduced, since the quantityof uranium and fission products which may leak out is smaller. The riskof secondary damage is also reduced for short rods.

To achieve the above-mentioned advantages, the number of fuel units ontop of each other may be at least three, preferably even more. To avoidusing spacers, the number of fuel units should be more than six.

What is claimed is:
 1. A nuclear light water moderated boiling waterreactor comprising: a plurality of fuel assemblies arranged verticallyin the reactor, the reactor being arranged such that water flows upwardsthrough said fuel assemblies whereby part of the water is transformedinto steam; at least one of the fuel assemblies comprising at least fivefuel units stacked on top of each other, each fuel unit comprising a toptie plate and a bottom tie plate and a plurality of fuel rods extendingbetween the top tie plate and the bottom tie plate; at least one of saidfuel units having fuel rods of different diameters from fuel rods ofanother of said fuel units; and said at least one fuel assembly having afuel channel with a substantially square cross section surrounding thestack of said at least five fuel units.
 2. A nuclear light watermoderated boiling water reactor according to claim 1, wherein at leasttwo of the fuel units in said at least one fuel assembly differ fromeach other in at least one of the following respects: the fuel unitshave different numbers of fuel rods; the fuel units have differentlattice configurations; the fuel units have different heights; the fuelunits have bottom tie plates with different embodiments with respect tothe mixing of coolant; and the fuel rods have different degrees ofenrichment.
 3. A nuclear light water moderated boiling water reactoraccording to claim 1, wherein said fuel units are retained by a springarrangement arranged in an upper part of the fuel assembly.
 4. A nuclearlight water moderated boiling water reactor according to claim 1,wherein at least one fuel rod in said at least one fuel assemblycomprises a top plug for attachment to the top tie plate of the fuelunit in which said at least one fuel rod is positioned and a bottom plugfor attachment to the bottom tie plate of said fuel unit in which saidat least one fuel rod is positioned, wherein, for receiving fissilegases, at least one of said top and bottom plugs is hollow and has anopening facing the interior of said fuel rod.
 5. A nuclear light watermoderated boiling water reactor according to claim 1, wherein at leastone fuel rod in said at least one fuel assembly has a coating protectingagainst abrasion damage to that part of the cladding tube of the fuelrod which is nearest the top tie plate of the fuel unit in which thefuel rod is positioned.
 6. A nuclear light water moderated boiling waterreactor according to claim 1, wherein each of said fuel units comprisesat least 80 fuel rod positions.
 7. A nuclear light water moderatedboiling water reactor according to claim 1 wherein the number of fuelunits in said at least one fuel assembly is larger than six.
 8. A methodcomprising: providing a fuel assembly in a nuclear light water moderatedboiling water reactor arranged such that water flows upwards throughsaid fuel assembly whereby part of the water is transformed into steam,said fuel assembly being arranged vertically in said reactor; providingat least five fuel units stacked on top of each other in said fuelassembly, each fuel unit comprising a top tie plate and a bottom tieplate and a plurality of fuel rods extending between the top tie plateand the bottom tie plate; providing, in at least one of said fuel units,fuel rods of different diameters from fuel rods of another of said fuelunits; wherein said fuel assembly has a fuel channel with asubstantially square cross section surrounding the stack of said atleast five fuel units.
 9. The method according to claim 8, wherein atleast two of the fuel units in said at least one fuel assembly differfrom each other in at least one of the following respects: the fuelunits have different numbers of fuel rods; the fuel units have differentlattice configurations; the fuel units have different heights; the fuelunits have bottom tie plates with different embodiments with respect tothe mixing of coolant; and the fuel rods have different degrees ofenrichment.
 10. The method according to claim 8, wherein said fuel unitsare retained by a spring arrangement arranged in an upper part of thefuel assembly.
 11. The method according to claim 8, wherein at least onefuel rod in said fuel assembly comprises a top plug for attachment tothe top tie plate of the fuel unit in which said at least one fuel rodis positioned and a bottom plug for attachment to the bottom tie plateof said fuel unit in which said at least one fuel rod is positioned,wherein, for receiving fissile gases, at least one of said top andbottom plugs is hollow and has an opening facing the interior of saidfuel rod.
 12. The method according to claim 8, wherein at least one fuelrod in said fuel assembly has a coating protecting against abrasiondamage to that part of the cladding tube of the fuel rod which isnearest the top tie plate of the fuel unit in which the fuel rod ispositioned.
 13. The method according to claim 8, wherein each of saidfuel units comprises at least 80 fuel rod positions.
 14. The methodaccording to claim 13, wherein the number of fuel units in said fuelassembly is larger than six.